In general, a database is any collection of information organized for rapid search and retrieval. A database stored in a computer-readable medium commonly is modeled as a collection of one or more tables. Each table, in turn, is modeled as a collection of one or more records (referred to commonly as a “row”), and each record as a collection of one or more fields (referred to commonly as a “column”). In a conventional table, all records comprise the same number and type of fields, and in the same order. A “relational” database consists of tables that are “related” to each other through common fields. The most common way to establish a relationship between two tables is to include one or more fields in each table that hold “key” information. A “primary key” field uniquely identifies a record, and commonly is just a number unrelated to other data in the record. Generally, a table can have only one primary key. A “foreign key” field is an identifier in a record that establishes a relationship with a primary key in another table. Unlike primary keys, though, a table can have an unlimited number of foreign keys establishing relationships with an unlimited number of tables. Thus, a primary key and foreign key combination effectively establish a “dependent” relationship, also known as a “parent-child” relationship, wherein the table having the primary key is the “parent” table, and the table having the foreign key is the “child” table.
The integrity of the relationships, though, depends on the foreign key referencing a valid primary key. A foreign key in a record of a child table loses all meaning if the corresponding primary key record in the parent table is unavailable. Thus, all foreign key values must have equivalent primary key values that already exist in the other table. A database management system (DBMS) must carefully manage the relationships between tables in order to provide users with consistent data and maintain referential integrity between tables. Particularly, if a user attempts to delete a record in the parent table without first deleting all corresponding records in the child table, the DBMS must make assumptions about the user's intent. One common assumption is that the user made an error, in which case the DBMS rejects the attempted deletion. Alternatively, the DBMS assumes that the user did not make an error, in which case the DBMS deletes all of the child records before deleting the parent records. In practice, a modern DBMS allows a user to designate foreign key fields and impose rules for managing relationships when the user creates the tables. A rule that causes a DMBS to delete all of the child records when the parent record is deleted is known commonly as a “cascading delete” or a “cascade-on-delete” rule.
For example, a mail order company that sells a variety of products would need to maintain information about their products, as well as orders for those products. The company could store this information in a database for rapid search and retrieval. In particular, the company probably would keep a catalog of available products that uniquely identifies each product, along with a description and the price of each product. The company also would need to keep track of each customer's order, including the customer's name and contact information, as well as the type and number of products ordered. Rather than putting all this information in one large table, though, the company would probably distribute the information in several smaller, related tables. A “PRODUCTS” table would represent the product catalog, and would include a primary key that uniquely identifies each record (thereby uniquely identifying each product). Similarly, an “ORDERS” table would include a primary key uniquely identifying each customer order. A third table, the “ORDER_ITEMS” table, would identify each item in a particular order. Each record in the ORDER_ITEMS table would include a foreign key linking the record to a specific order in the ORDERS table, and a foreign key linking the record to a specific product in the PRODUCTS table. The PRODUCTS and ORDERS tables would be examples of parent tables, while the ORDER_ITEMS table would be an example of a child table. Because the items in each order are represented in a separate table, each order can have a variable, unlimited number of items. But if a customer cancels an order, both the record representing the order and the records representing the items in the order should be deleted. If the ORDERS and ORDER_ITEMS tables include a cascading delete rule, then the DBMS will automatically find the order's child records and delete them before deleting the order.
For many years, computer programmers created client programs that interacted directly with a DBMS server program. In general, a client program provided a user interface to the DBMS and implemented an enterprise's business rules for a particular application, such as entering a customer's order for products. The enterprise data, though, remained relatively centralized and available to multiple clients through the DBMS server program. In recent years, this traditional “two-tier” client/server system has been displaced slowly by more sophisticated multi-tier client/server systems. In general, a multi-tier system places at least one intermediate component between the client and the server. These components are referred to commonly as “middleware.” Generalized “n-tier” systems include n layers of software that provide a different layer of services at varying levels of detail to the layers above and beneath them, where n is any number. See Mark Johnson, A beginner's guide to Enterprise JavaBeans, JavaWorld, at http://www.javaworld.com (October 1998), incorporated herein by reference. Programmers often use multiple client/server tiers to separate and delegate particular programming tasks. In particular, one tier usually provides a user interface, while a second tier implements an enterprise's business rules. A third tier generally implements the underlying data processing (such as creating a data structure to represent a customer's order in a database).
“Object-oriented” languages and techniques also have become increasingly popular in recent years. In general, an “object” is a named memory unit that contains data and instructions for manipulating that data. In an object-oriented context, the term “attribute” or “property” generally refers to the data within the memory unit, and the term “method” or “procedure” refers to the related instructions for manipulating the data. In practice, objects often include methods that direct the process of storing the object's attributes within a file or database. Of course, an object that includes such a method also generally includes one or more methods that direct other types of operations, such as updating or removing the object in the file or database.
Today, computer programmers frequently build application programs with a mix of n-tiered architectures and object-oriented technology. Sun Microsystems, Inc. (SUN) has developed a comprehensive collection of objects and other supporting programs that programmers can use to build sophisticated transaction processing systems. SUN markets this collection as the JAVA 2 ENTERPRISE EDITION (J2EE) platform. SUN also has developed an application program interface (API) for J2EE that defines an n-tiered architecture, which SUN markets as the ENTERPRISE JAVABEANS (EJB) architecture.
Generally, an EJB architecture comprises an EJB server, an EJB container, an EJB component (also commonly known as a “bean”), an EJB object, and a database. FIG. 1 depicts a typical EJB system architecture. An EJB component, which typically implements business rules, executes within an EJB container. The EJB container, which implements many of the data processing operations, executes within an EJB server. The EJB server generally executes within any given computer's native environment and provides the EJB container access to low-level computing resources. An EJB object allows a client program to execute the EJB component, through the EJB component's EJB container. Generally, each of these EJB subsystems comprises one or more objects that implement the functions of the subsystem. Thus, the term “EJB client” will be used herein, instead of the term “EJB object,” to avoid any confusion with a generic “object.”
An “entity bean” is one type of EJB component used to model data in business transactions, the attributes of which are typically stored within a database. The term “persist” generally refers to the process of storing, updating, and deleting such attributes to or from a database. An entity bean may manage the persistence of its attributes, or it may delegate the responsibility to the EJB container in which it executes. An EJB client may explicitly request the entity bean, or the EJB container, to persist the entity bean's attributes. Alternatively, the entity bean or EJB container, as the case may be, may persist the attributes when there is a need, such as occurs when a second EJB client needs to access the attributes in the database.
An entity bean also may have a relationship with one or more other entity beans, much like a table in a database may have a relationship with other tables. Entity bean relationships must be managed much as table relationships must be managed, particularly to maintain referential integrity. In general, an EJB container is responsible for managing bean relationships. Consequently, such a relationship is referred to commonly as a “Container-Managed Relationship” (CMR). An EJB container generally persists a CMR in a database as one or more foreign keys referencing one or more primary keys.
Persons skilled in the art will appreciate that any operation that accesses a database consumes at least some quantity of available computing resources, thereby decreasing the resources available for other computing tasks. Thus, a computer program that frequently accesses a database can decrease computer performance significantly. Likewise, a computer program that accesses a database indirectly through one or more objects, such as an entity bean or container, can cause the same performance reduction.
Currently, in an EJB architecture, and in object-relational-database persistence systems generally, an EJB container and the DBMS system are both responsible for maintaining referential integrity in their respective domains. But if objects are persisted as tables in a database and relationships between objects are mapped with foreign keys, then many of these responsibilities are duplicative and unnecessarily consume valuable computing resources.
Thus, there is a need in the art for a uniform process for minimizing the number of times that a computer program accesses a database to maintain persistent referential integrity. The invention described in detail below addresses this need. This and other objects of the invention will be apparent to those skilled in the art from the following detailed description of a preferred embodiment of the invention.